Cracking Tendency of High Strength Lightweight Aggregate Concrete at Early Ages

The use of high strength normal weight and lightweight aggregate concrete (i.e., with water/binder ratios below 0.40) have shown that the concrete may be more sensitive to cracking the first hours and days after casting (due to autogenous shrinkage and thermal strains) than normal strength concretes. Two test rigs have been built in order to investigate the problem. The "Shrinkage-Rig" determines the "free" deformations (e.g., autogenous and thermal deformations), and the special "Stress-Rig" determines the stresses when the concrete is restrained against the deformations. The paper presents the results from testing of one high strength normal weight concrete and one high strength lightweight aggregate concrete, both with water/binder ratio 0.38, in the test rigs. Both concretes were exposed to two different temperature histories generated from heat of hydration. The normal weight concrete developed relatively high tensile strains during cooling phase. The corresponding stresses in the Stress-Rig became very high, and in one case the concrete failed. The lightweight aggregate concrete, however, did not develop any tensile strain, due to a lack of autogenous shrinkage caused by the water supply from the lightweight aggregate grains. Consequently, no severe tensile stresses were built up in the Stress-Rig.